CONTRIBUTING.md file. Update slack sign-up link

Signed-off-by: Pavel Sirotkin <[email protected]>

CONTRIBUTING.md

@@ -83,7 +83,7 @@ don't get discouraged! Our contributor's guide explains
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     <td>Community Slack</td>
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-      The Docker Community has a dedicated Slack chat to discuss features and issues.  You can sign-up <a href="https://dockercommunity.slack.com/ssb/redirect" target="_blank">with this link</a>.
+      The Docker Community has a dedicated Slack chat to discuss features and issues.  You can sign-up <a href="https://www.docker.com/docker-community" target="_blank">with this link</a>.
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